Pulse-echo acoustic ranging systems, also known as time-of-flight ranging systems, are commonly used in level measurement applications. Pulse-echo acoustic ranging systems determine the distance to a reflector (i.e. reflective surface) by measuring how long after transmission of a burst of energy pulses the echo or reflected pulses are received. Such systems typically use ultrasonic pulses or pulse radar or guided radar signals.
Pulse-echo acoustic ranging systems generally include a transducer and a signal processor. The transducer serves the dual role of transmitting and receiving the energy pulses. The signal processor is for detecting and calculating the distance or range of the object based on the transmit times of the transmitted and reflected energy pulses.
When measuring distance using the time of flight method in radar or TDR (time domain reflectometry) based instruments, it is difficult to maintain a high degree of accuracy over a range of temperatures and operating conditions. There will be variations in the electronic components that lead to errors in the measured distance.
In the art, the method of sampling multiple pulses to create a lower speed representation is known. It typically involves using two pulse trains at slightly different frequencies with the higher frequency clock used to generate transmit measurement pulses and the lower frequency clock is used to generate sampling pulses. The control of these pulses is performed by a timebase generator and it is critical to create a precisely known start instant and apparent velocity in order to precisely determine the distances. Systems have been designed which control the frequencies very well, but fall short of accommodating variations in the high frequency pulsing sections which are driven by the timebase generator.
A radar or TDR system can be built and calibrated according to the art to be accurate to a few millimeters which is equivalent to about 7 picoseconds per millimeter. However, as a result of aging or temperature variation in the components, there may be slight differences in the operation of the radio frequency (RF) oscillators or in the pulses (i.e. control signals) that enable the RC oscillators. In addition, the timebase generator may itself introduce a few picoseconds of error that can add or subtract from the total accumulating error. While these variations can be minimized by further design enhancements, the design becomes more and more expensive as higher and better accuracy and stability is sought, until the point is reached where it is prohibitively expensive to improve the accuracy any further.
Accordingly, there remains a need for new methods and apparatus to improve the accuracy of such systems.